1. Field of the Invention
This invention relates generally to location receivers and more particularly to phase locked loop (PLL) synthesizer employed in location receivers.
2. Related Art
A Global Navigation Satellite System (GNSS), such as a GPS, Galileo, or GLONAV satellite system is based on radio navigation. The GPS system is a satellite based navigation system having a network of 24 satellites, plus on orbit spares, orbiting 11,000 nautical miles above the Earth, in six evenly distributed orbits. Each GPS satellite orbits the Earth every twelve hours.
A prime function of the GPS satellites is to serve as a clock. Each GPS satellite derives its signals from an on board 10.23 MHz Cesium atomic clock. Each GPS satellite transmits a spread spectrum signal with its own individual pseudo noise (PN) code. By transmitting several signals over the same spectrum using distinctly different PN coding sequences the GPS satellites may share the same bandwidth without interfering with each other. The code used in the GPS system is 1023 bits long and is sent at a rate of 1.023 megabits per second, yielding a time mark, sometimes called a “chip” approximately once every micro-second. The sequence repeats once every millisecond and is called the coarse acquisition code (C/A code). Every 20th cycle the code can change phase and is used to encode a 1500 bit long message, which contains “almanac” data for the other GPS satellites.
There are 32 PN codes designated by the GPS authority. Twenty-four of the PN codes belong to current GPS satellites in orbit and the 25th PN code is designated as not being assigned to any GPS satellite. The remaining PN codes are spare codes that may be used in new GPS satellites to replace old or failing units. A location receiver, such as a GPS receiver, may use the different PN sequences to search a received signal spectrum looking for a match. If the location receiver finds a match, then it has identified the GPS satellite, which generated that signal.
Location receivers typically use a variant of radio range measurement methodology, called trilateration, in order to determine the position of the ground based location receiver. The position determination employed by a location receiver is different from the radio direction finding (RDF) technology of the past in that the radio beacons are no longer stationary; they are satellites moving through space at a speed of about 1.8 miles per second as they orbit the earth. By being space based, the GPS system can be used to establish the position of virtually any point on Earth using methods such as trilateration.
The trilateration method depends on the location receiving unit obtaining a time signal from the GPS satellites. By knowing the actual time and comparing it to the time that is received from the GPS satellites, the location receiver can calculate the distance to the GPS satellite. If, for example, the GPS satellite is 12,000 miles from the location receiver, then the location receiver must be located somewhere on the location sphere defined by the radius of 12,000 miles from that GPS satellite. If the location receiver then ascertains the position of a second GPS satellite it can calculate the receiver's location based on a location sphere around the second GPS satellite. The two spheres intersect and form a circle with the location receiver being located somewhere within that location circle. By ascertaining the distance to a third GPS satellite the location receiver can project a location sphere around the third GPS satellite. The third GPS satellite's location sphere will then intersect the location circle produced by the intersection of the location spheres of the first two GPS satellites at just two points. By determining the location sphere of one more GPS satellite, whose location sphere will intersect one of the two possible location points; the precise position of the location receiver is determined to be the location point located on the Earth. The fourth GPS satellite is also used to resolve the clock error in the receiver. As a consequence, the exact time may also be determined, because there is only one time offset that can account for the positions of all the GPS satellites. The trilateration method may yield positional accuracy on the order of 30 meters; however the accuracy of GPS position determination may be degraded due to signal strength and multipath reflections.
As many as 11 GPS satellites may be received by a location receiver at one time. In certain environments such as a canyon, some GPS satellites may be blocked out, and the GPS position determining system may depend for position information on GPS satellites that have weaker signal strengths, such as GPS satellites near the horizon. In other cases overhead foliage may reduce the signal strength that is received by the location receiver unit. In either case the signal strength may be reduced or totally blocked. In such case, aiding information may be used to aid in location determination.
There are multiple ways of using radio spectrum to communicate. For example in frequency division multiple access (FDMA) systems, the frequency band is divided into a series of frequency slots and different transmitters are allotted different frequency slots. In time division multiple access (TDMA) systems, the time that each transmitter may broadcast is limited to a time slot, such that transmitters transmit their messages one after another, only transmitting during their allotted period. With TDMA, the frequency upon which each transmitter transmits may be a constant frequency or may be continuously changing (frequency hopping).
As previously mentioned, another way of allotting the radio spectrum to multiple users is through the use of code division multiple access (CDMA) also known as spread spectrum. In CDMA all the users transmit on the same frequency band all of the time. Each user has a dedicated code that is used to separate that user's transmission from all others. This code is commonly referred to as a spreading code, because it spreads the information across the band. The code is also commonly referred to as a Pseudo Noise or PN code. In a CDMA transmission, each bit of transmitted data is replaced by that particular user's spreading code if the data to be transmitted is a “1”, and is replaced by the inverse of the spreading, code if the data to be transmitted is “0”.
To decode the transmission at the receiver unit it is necessary to “despread” the code. The despreading process takes the incoming signal and multiplies it by the spreading code chip by chip and sums the result. This process is commonly known as correlation, and it is commonly said that the signal is correlated with the PN code. The result of the despreading process is that the original data may be separated from all the other transmissions, and the original signal may be recovered. A property of the PN codes that are used in CDMA systems is that the presence of one spread spectrum code does not change the result of the decoding of another code. The property that one code does not interfere with the presence of another code is often referred to as orthogonality, and codes, which have this property, are said to be orthogonal. The process of extracting data from a spread spectrum signal is commonly known by many terms such as correlating, decoding, and despreading. Those terms may be used interchangeably herein. The codes used by a spread spectrum system are commonly referred to by a variety of terms including, but not limited to, PN (Pseudo Noise) codes, PRC (Pseudo Random Codes), spreading code, despreading code, and orthogonal code. Those terms may also be used interchangeably herein.
It is because CDMA spreads the data across a broadcast spectrum larger than strictly necessary to transmit data that CDMA is often referred to as spread spectrum. Spread spectrum has a number of benefits. One benefit being that because the data transmitted is spread across the spectrum, spread spectrum can tolerate interference better than some other protocols. Another benefit is that messages can be transmitted with low power and still be decoded, and yet another benefit is that several signals can be received simultaneously with one receiver tuned on the same frequency.
The GPS system uses spread spectrum technology to convey its data to ground units. The use of spread spectrum is especially advantageous in satellite positioning systems. Spread spectrum technology enables location receiver units to operate on a single frequency, thus saving the additional electronics that would be needed to switch and tune other bands if multiple frequencies were used. Spread Spectrum also minimizes power consumption requirements of location receivers. GPS transmitters for example require 50 watts or less and tolerate substantial interference.
The location receivers are generally composed of a radio frequency (RF) front end portion and a baseband portion that are formed using integrated circuits. These integrated circuits are implemented in bipolar, BiCMOS, or CMOS process technology. The Front-end portion of a typical location receiver downconverts the GPS RF input signal to a lower intermediated frequency (IF) signal, by mixing the GPS RF with an RF local oscillator (LO) signal which is relatively close in frequency to the GPS RF frequency. Typically the front end also includes a phase locked loop (PLL) synthesizer for locking the LO frequency to a reference frequency. The RF/analog PLL does not match to the phase of the incoming signal; the incoming GPS signal is buried in noise and it is only after the correlation/despreading process in the baseband that the signal phase may be recovered. Matching to the PRN code phase is done digitally in the baseband portion. The PLL synthesizer also provides the clock for the analog-to-digital converter (ADC) which digitizes the downconverted IF signal. The PLL synthesizer may be made up of components that include a fractional-N divider, integer divider, and phase frequency detector (PFD). The logical functions in the dividers and PFD are typically implemented in Emitter Coupled Logic (ECL) for bipolar technology or Source Coupled Logic (SCL) for MOS technology. These implementations require dc current biasing, 3 transistor stacks, and resistors or a 4th transistor level for the load, all of which limit the size, supply voltage, and power consumption of these circuits. Implementation in CMOS logic, has been avoided because of the supply and ground noise generated during rail-to-rail transitions.
The PLL synthesizer in existing location receiver integrated circuits typically consumes 30% to 50% of the front-end power, and a larger portion of the integrated circuit area. Thus, the PLL synthesizer has a significant influence on the size, power consumption and silicon cost of the location receiver. One of the reasons for the high power requirements of the PLL synthesizer is the dc current required by the ECL or SCL technology used to implement the components of the PLL synthesizer.
Therefore, there is a need for methods and systems for implementing PLL synthesizer in an integrated circuit, such as used in location receivers that reduce the area in the integrated circuit required by the PLL synthesizer and a reduce the power requirements of the integrated circuit. This is increasingly important for hand-held and portable applications of GPS receivers.